1. Field of the Invention
The invention relates to an integrated circuit power-on/power-off checking device, namely a device to check power-on and power-off operations in an integrated circuit.
2. Description of the Related Art
A power-on/power-off checking device monitors the level of the supply voltage to enable, if necessary, the resetting of certain electronic elements such as sequential circuits and prevent access to certain electronic functions. The checking device therefore fulfills a detection function that is very important for the operating security of the integrated circuit, enabling the activated integrated circuit to be in a clearly determined state (a resetting state) and making it possible, when there are conditions of supply that do not comply with specifications, to prevent access to certain functions in order to preserve the integrity of the integrated circuit.
To obtain a high level of security, the common practice is to use a checking circuit with a voltage reference circuit enabling the detection of the level of the supply voltage with respect to a threshold voltage Vs established by the voltage reference circuit. A checking device of this type is described, for example, in the French patent application No. 96 01378.
A device of this kind includes a circuit for current biasing the voltage reference circuit, enabling the permanent detection of the level of the supply voltage. The point of connection between the voltage reference circuit and the bias circuit gives a detection signal capable of causing the switch-over of the inverters of an output stage. This output state delivers a neutralization or power-on reset signal POR that is applied to a resetting and inhibition circuitry of the integrated circuit.
As shown schematically in FIG. 1, this neutralization signal POR follows the level of the supply voltage Vdd so long as this level is below the detection threshold Vs. When the level is greater than the detection threshold Vs, the signal POR is drawn to zero volts.
When the signal POR follows the supply voltage Vdd, it is perceived as a "1" by the resetting and inhibition circuitry which is then activated. It will be noted however that this can be done only when the supply voltage Vdd has a sufficient level, at least equal to a level Vf, at which the circuitry can function. The detection threshold Vs is necessarily greater than this minimum level Vf. It is in the period of time when the voltage Vdd is between Vs and Vf that the resetting is done and that the locking of certain access paths is done through the pulse of the signal POR. As soon as the supply voltage Vdd is greater than the detection threshold Vs, the resetting and inhibition circuitry is deactivated and the integrated circuit is operational, reset and has all its electronic functions valid.
The constant current bias of the voltage reference circuit leads to permanent power consumption in the integrated circuit, even in standby mode, when the integrated circuit is not selected. In this standby mode, this consumption is highly inconvenient because it is greater than the consumption in standby mode of the rest of the integrated circuit. As an order of magnitude, the consumption is, for example, in the range of 10 micro-amperes for the control device alone, and some nano-amperes for the rest of the integrated circuit. When the integrated circuit is selected, this consumption of the control device becomes negligible with respect to the consumption of the other electronic functions of the circuit.